Magnified observation of chloride-induced corrosion at the steel-concrete interface

Please check whether enrolling in this project is possible by directly contacting the responsible supervisor(s) indicated below as soon as possible. Note that although we announce many topics, we won't be able to offer all of them simultaneously due to limited lab capacity.

The necessity to reduce the risk of failure of reinforced concrete structures and to reduce the production of concrete, which is associated with a large formation of CO₂, opened many questions concerning the material degradation mechanisms. The most dangerous mechanism of deterioration is represented by corrosion of steel in concrete, which leads to the reduction of the steel cross section over time and eventually to the failure of the structural element. The most dangerous form of corrosion is chloride-induced corrosion which, although localized, normally causes the very fast loss of cross-section of the steel reinforcement. The output of this process is dangerous and well known, while many unanswered questions are still present in literature regarding the reactions and mechanisms leading to the initiation of chloride-induced corrosion.

The project here proposed aims to reproduce the conditions present at the steel-concrete interface (SCI), in case of reinforced concrete exposed to chloride bearing environments (e.g. marine environment or de-icing salts spread on infrastructures) considering different saturation degrees of concrete. The goal is to observe the location of the corrosion attack in the different scenarios. The project is mostly practical (performed in laboratory) and it will imply the realization of “magnified” and non-realistic examples of SCI, as shown in Fig.1.

experimental setup — Fig.1. Samples representative of the condition at the SCI

Each sample will be made of a carbon steel rebar (diameter 6 mm, length 50 mm) surrounded by two concrete walls (50 x 40 x 10 mm). The edges of the steel rebars will be coated with epoxy resin in order to avoid the occurrence of corrosion in these areas, and the bars will be previously passivated in an alkaline solution. Between the two concrete walls, a cavity is formed and sealed with glass/Plexiglas sheets. This cavity will contain air and a solution reproducing the pore solution normally present in concrete with the addition of sodium chloride (NaCl), in order to trigger corrosion. The amount of solution will vary among the different specimens and the exposure conditions (e.g. wet-dry cycles or 100% submersion) as well. The conditions inside of the cavity could be assessed through relative humidity (RH) sensors and the polarization potential of the rebar could also be monitored over time. After corrosion onset, assessed through direct visual observation, evaluations of the corrosion attack will be made considering the boundary conditions to which the sample was exposed. The output of the project contribute to elucidate the mechanisms of chloride-induce corrosion of steel in concrete.